Eyyy, it's Pound and Pumpkin's little sister, Party Cake! She was named by Pinkie because her parents couldn't agree on a name an Pinkie was practically part of the family at that point (pretty sure she's been part of the family since the beginning but whatever) and she's pretty strong for her age and size.
A select few of you may recognise Part Cake. Why? Well, because she's a G3 that I've re-purposed; I've not seen her used in the show at all and I thought that I might as well make use of her design. Personally, I think her name and colours fit pretty well with Mr and Mrs Cake.
Party is close to her older brother and the two often hang out together, but she's nowhere near as close with her older sister; Pumpkin wasn't too pleased with having a little sister, and so stayed away from the baby. When she grew up a bit, she realised she was being stupid, but by that point, she didn't know how to approach her sister. Party tried when she was little, but mostly gave up after a while. She still tries, but the interactions are awkward and so come few and far between. They're less like sisters and more just ponies that live together.
Don't really have that much to say about her now, hope you like her